D/A interface modification 25

1.4.1.1 Background and development of interface nanostructuring

Light absorption and exciton formation ηabs can be enhanced by either improved material dependant spectral overlap with the solar spectrum but also an increased film thickness. In order to perform exciton dissociation or charge transfer ηct, the generated excitons need to reach a suitable heterojunction before recombination. This process is dominated and limited by the short LD leading to a low exciton diffusion efficiency ηed.

Chapter 1: Introduction Therefore a compromise between the film thickness for maximum absorbance and the limited LD for efficient exciton dissociation is essential for efficient photocurrent generation. This relationship defines the main limiting factor of planar bilayer OPV devices as shown in Figure 1.9a.[26]

Figure 1.9 Schematic of different heterojunction OPV device architectures: a) bilayer, b) BHJ or mixed

layer, c) and d) 3D nanocomposite devices. The black and red arrows indicate the traveling path of electrons and holes respectively. In b) (i) is a situation of a charge trapped in a dead end and (ii) shows successful charge transport to the collecting electrodes after exciton splitting.

To minimise the exciton diffusion paths and to generate a larger interface with increased total active layer thickness, D/A intermixed active layers were introduced, including solution-processed polymer/fullerene BHJ devices and vacuum co-deposited small molecules. In both cases charge transport pathways towards the charge collecting electrodes through randomly mixed layers are limited due to numerous isolated domains and cul-de-sacs in the D/A layer system resulting in charge trapping and recombination, as highlighted in Figure 1.9b. Despite the current density increase in both BHJ and mixed layer structures, there is an unavoidable trade-off between the improved ηedand reduced ηcc.[26, 135] By optimising the deposition conditions and by applying post-treatments such as temperature and solvent annealing a certain control over phase segregation and therefore the D/A interface order can be achieved.[136, 137] A theoretical study by Yang et al. on photo-current generation in nanostructured OPVs revealed exactly the same trend. As the domain size of a D/A mixed layer was increased the specific interface area in a defined unit cell dropped resulting in an improved ηcc but reduced ηed and a small, but still noticeable improvement in internal quantum efficiency (IQE).[132]

Splitting exciton Hole movement

Electron movement

(i) (ii)

To overcome this problem a more controlled three-dimensional (3D) highly interpenetrating D-A composite structure is required to exploit the advantages from BHJ and mixed layers, but with well structured charge transport paths.

Potential solutions for this complex problem are ordered organic nanostructures which would result in an increase in interface area, and therefore short exciton diffusion pathways, but also continuous charge transport pathways with increased film thickness and therefore improved absorbance.

An ideal solution is a finger-shaped interdigitated D-A device architecture with a small diameter to meet the LD criterium.[138, 139] Such structures have been realised in hybrid devices from vertically aligned metal oxide nanorods, but the devices showed only slight device current improvement.[140-142] Another promising attempt by Haberkorn et. al. is the template-assisted fabrication of free-standing nanorod arrays of a hole- conducting crosslinked triphenylamine derivative.[143] _{A new route to achieve such} ordered D/A interface patterning on a length scale of a few tens of nanometres in domain size is the use of self-assembled block copolymers facilitating donor and acceptor domains in the same chain. This route is complex from a synthetic and self-assembly point of view and remains very challenging.[144] However, such a finger-shaped interpenetrating D/A system is not easy to realise on a sub-100 nm scale for purely organic OPVs (Figure 1.9c).

Close approximations to interpenetrating nanostructured interfaces have been produced by nanosphere lithography (NSL).[145] NSL has been used to generate nanosphere templated 2D nanocomposite organic thin film structures based on a nanoparticle monolayer mask as a template, which consists of 2D-ordered nanosphere arrays. However, the interface area would be greatly compromised compared to any mixed or BHJ interface.

To take this development one step further template assisted three-dimensionally ordered macroporous solids (3DOM) and open-cellular thin films of the appropriate organic semiconductor could form the desired controlled matrix for ordered highly interpenetrating D-A composite systems, as demonstrated in Figure 1.9d. A synthetic opal structure from self-assembled polystyrene nanospheres, fabricated by sedimentation, spin-coating or controlled vertical drying, can serve as the initial template. The

Chapter 1: Introduction fabrication process of 3DOM thin films typically involves three separate steps: (i) self- assembly of colloidal spheres or droplets into supra-structures; (ii) infiltration of the interstitial spaces with an application-specific material; and (iii) template removal. In certain cases the first two steps are combined into a single co-deposition procedure. The nanosphere templating process is widely applicable to inorganic materials including metal oxides and metals but proves to be very challenging for organic semiconductors.[146] Further details are revealed in Chapter 5.

1.4.1.2 Concept and fabrication strategies

3D nanosphere templating involves numerous steps and processes to obtain the highly interpenetrating D/A composite structure: 1) convective self-assembly of polystyrene colloids to form the template structure, 2) infiltration of the nanosphere domains with appropriate donor material, which can be combined to a direct co- deposition, 3) colloid removal step, 4) second infiltration of the inverse structure with acceptor material, and 5) deposition of buffer layer and vacuum deposition of the covering top electrodes. The schematic of nanosphere templating for a complete nanocomposite device is shown in Figure 1.10.

Figure 1.10 Schematic of fabrication method: 1) Self-assembly of polystyrene colloids, 2) infiltration of

nanosphere domains with appropriate donor material, 3) colloid removal, 4) infiltration of inverse structure with acceptor material, 5) device fabrication using 3D nanosphere templating combined with organic molecular beam deposition. 1) and 2) can be combined to a co-deposition step.

In this unique approach to template organic semiconducting materials the templating material is sacrificial, which explains the choice of polystyrene (PS). The system is based on a two-phase system starting with water as a solvent and dispersion medium for PS and the water-soluble donor materials (PTEBS and TSCuPc). The removal process of the PS template by using non-polar solvents is selective leaving the

2 3 5

inverse opal structure made of donor material unchanged. The second infiltration of the acceptor material has to be performed from a non-polar solvent to prevent the remaining structure from damage or even complete dissolution. In this delicate approach it is of great importance to create a clean D/A interface avoiding any residues from the performed process steps and more importantly from the template. PS is a very good insulator and a thin film could already ruin the device without being detected. Other sources of residues are soaps which are added to stabilise the nanospheres during and after synthesis. Soaps and other additives are also the reason why all nanospheres were synthesised in house to control all parameters and compounds involved in the synthesis.

Co-deposition was developed to target very small sphere sizes down to 50 nm in diameter in order to match LD, and is completely new to the field. The templating approach can also be used to template TiOx and ZnO for nanostructured electrodes or hybrid devices.